This invention relates to a demodulator for demodulating an input signal subjected to amplitude-and-phase modulation.
With the amplitude-and-phase modulation technique, it is possible to transmit a plurality of digital signal series which are independent of one another. As will later be described with reference to a figure of the accompanying drawing, an amplitude-and-phase modulated signal includes, besides quadrature phase components spaced in the modulated signal by a 2.pi./4 radian phase interval, a plurality of additional components. In a conventional amplitude-and-phase demodulator, the amplitude-and-phase modulated signal supplied thereto as an input signal is demodulated by the use of a pair of reference carrier signals, one of which is in quadrature phase relative to the other and which are recovered from the input signal. An error component inevitably remains in the recovered reference carrier signals due to the additional components. Consequently, the conventional demodulator is defective in that jitters are inevitable in the recovered reference carrier signals although the circuitry for recovering the reference carrier signals is complicated in structure to render the demodulator expensive. As a further result, the conventional demodulator has been incapable of reproducing the digital signal series with high precision.